Organized in 1946, Daniel Mann Johnson and Mendenhall (DMJM) was one of the first combined architecture and engineering firms in the Western U.S. Opened at a prosperous moment for architects in a booming section of the US, DMJM grew rapidly, becoming one of Southern CA's largest full-service design and engineering firms by the 1960s. By 1970, DMJM had 14 offices across the US, including those in Los Angeles, CA, Honolulu, HI, Portland, OR, Redwood City, CA, Washington, D.C., Las Vegas, NV, and in 1970; at this time, other Partners in DMJM in 1970 included the noted designers Anthony Lumsden and Cesar Pelli. DMJM also operated a branch office in Baltimore, MD, c. 1973. In 1978, Louis C. Milani served as Manager of the DMJM construction management division, Technical Management Services, Incorporated.

In 1984, Ashland Oil, Incorporated, of Covington, KY, purchased DMJM and held it for six years. It sold the company back to its employees, who formed the firm AECOM in 1990. AECOM grew dramatically by further acquisitions and, because of its now far-flung construction, construction management, and engineering operations reorganized its subsidiaries in 2000. Portions of DMJM became parts of two AECOM companies: DMJMH Harris and DMJM Holmes and Narver. DMJM Harris became based in New York, NY, with several West Coast branch offices, focused on transportation and infrastructure design and construction projects. DMJM Holmes and Narver remained based in Los Angeles, and had a general design focus working for mainly institutional, corporate and governmental clients. In 2006, AECOM had 1,171 employees in its DMJM-branded subsidiaries.